Tip One: Use Fire, Steel, Rock of Fighting Pokémon.
Tip Two: Do not use Ice Pokémon.
I would suggest getting powerful Pokémon with types supereffective on Glalie. These include Klefki, Mega Mawile, and Combusken. I would also suggest trying to get Expert Stage Pokémon like Lucario, Charizard and Moltres, who do higher-than-average damage. If you have enough spare coins go for some Complexity -1's or Mega Starts.